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Chapter 15

Chapter 15. Firearms, Tool Marks, and Other Impressions. Objectives:. Describe techniques used to lift impressions such as tire tracks and shoeprints. Distinguish between class and individualization of evidence from firearms and bullets. Firearm Identification.

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Chapter 15

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  1. Chapter 15 Firearms, Tool Marks, and Other Impressions

  2. Objectives: • Describe techniques used to lift impressions such as tire tracks and shoeprints. • Distinguish between class and individualization of evidence from firearms and bullets.

  3. Firearm Identification • The high frequency of shooting cases has necessitated the science of firearm identification extend beyond the mere comparison of bullets. • Knowledge of operation of all types, restoration of serial numbers, detection & characterization of GSR, estimation of muzzle to target distances, etc.

  4. Bullet Comparisons • The inner surface of the barrel of a gun leaves its markings on a bullet passing through it. • Unique to each gun.

  5. Making a Gun • Hollowed out by drilling. • Impression of spiral grooves called rifling. • The area between the grooves are called lands. • Puts the bullet into a rapid spin. • The interior surface of the firearm barrel is called the bore.

  6. Caliber of a Gun • The caliber is the diameter of the gun barrel, measured between opposite lands. • Recorded in hundredths of an inch or millimeters (ex: .22, .38, and 9 mm) • Caliber is not an exact measurement of the barrel’s diameter.

  7. Class Characteristics • Every firearms manufacturer chooses a rifling process best suited for its product. • Once the choice is made, the class characteristics are consistent for that brand. • Permits distinguishing between one type or brand-name weapon from another. • Does not impart individuality to any one barrel.

  8. Striations • Fine lines running the length of the barrel’s lands and grooves. • Impart individual characteristics to a gun. • No two rifled barrels, even those manufactured in succession, will have identical striations.

  9. The Effect On a Bullet • As the bullet passes through the barrel, its surface is impressed with the rifled markings of the barrel • These impressions carry both class and individual characteristics of the barrel.

  10. The Indispensable Tool • Comparison Microscope • Single most important tool for the firearm examiner.

  11. Bullet Examination Rules • No situation is perfect. • Grit, rust, etc. may alter the bullet’s markings. • Bullets recovered may also be mutilated or distorted. • Striation changes may occur over time. • There are no rules governing the minimum number of points required for a bullet comparison.

  12. Exceptions • Some weapons have rifling marks that set them apart form most other manufactured weapons. • Ex: Marlin rifles • Contain microgrooves. • The FBI maintains a record known as the General Rifling Characteristic File on known weapons.

  13. Shotguns • Shotguns have a smooth barrel. • Projectiles passing through will not be impressed with any markings. • Shotguns generally fire small lead balls or pellets.

  14. Shotguns • The diameter of the shotgun barrel is called the gauge. • The weight and measurement of the shot recovered or the size and shape of the wad may reveal the gauge of the shotgun • The higher the gauge number, the smaller the barrel’s diameter. • The exception is a .410.

  15. Distinctive Markings of Shells and Cartridges • May be made by: • Firing pin • Breechblock – rear part of barrel • Extractor and ejector mechanism • Note: • Firing pin – primer- ignites powder – expand gases – propel bullet – pushes spent cartridge backward against breechblock

  16. Chokes • Degree of constriction placed at the muzzle end of the barrel. • The greater the choke, the narrower the shotgun pattern will be and the faster and farther the pellets will travel.

  17. Automated Firearm Search Systems • FBI – DRUGFIRE • Analysis on markings on cartridge casings expelled by firearms • BATFE – IBIS • markings on expended bullets and casings. • Only a screening tool • Final determination made by firearm examiner • FBI & ATF – NIBIN • Program that provides firearm analysis guidelines and offers assistance in housing an automated system. *The new IBIS system incorporates both DRUGFIRE and IBIS technology available in prior years.

  18. The Future • Ballistic Fingerprinting • System that would capture and store appropriate markings on bullets and cartridges test fired from handguns and rifles before they are sold to the public.

  19. Gunpowder Residue • Necessary to determine firing distance in order to rule out self defense plea. • The distribution of gunpowder particles and other discharge residues around the bullet hole permits an assessment of the distance from which a handgun or rifle was fired. • See page 445, Figure 15-12

  20. Firing Distances • Less than 1 inch • heavy concentration of smokelike vaporous lead • Scorch marks or signs of fabric melting • Stellate pattern • 12 to 18 inches or less • Halo of vaporous lead deposited around bullet hole • Scattered specks of unburned or partially burned powder • More than 3 feet • Usually no deposition of powder residue • Bullet wipe – dark ring around bullet hole ** The spread in the pattern made by a 12 gauge shotgun is increased 1 inch for each yard of distance.

  21. Powder Residue on Garments • First examine microscopically. • Infrared photography • Best to use if color of garment is an issue • Chemical tests • Greiss Test

  22. Primer Residues on the Hands • Dermal nitrate test • Fallen out of favor • Positives not always indicative of using a firearm. • Fertilizers, cosmetics, urine, tobacco, etc. • Neutron Activation Analysis • Flameless Atomic Absorption Spectrophotometry • SEM Primer Residue Detection – most promise for future GSR detection

  23. Primer Residues • Barium and Antimony • Major difficulty appears to be the short time the primer residues remain on the hands. • Will not accept hand swabs from suspects taken 6 or more hours after firing has occurred.

  24. Gathering of Weapon Evidence • Underwater • Must be packaged in a container with enough of the liquid to cover the weapon. • Then transport to crime lab.

  25. Obliterating Serial Numbers • Generally doesn’t completely remove the stamped region. • An etching agent can be applied to restore the imprint. • The stamped area will dissolve at a faster rate than the unstamped area indicating the serial number.

  26. Packaging Firearm Evidence • Unload weapon before shipping • Avoid sticking objects into the barrel of a weapon to pick it up • Label undischarged bullets by chamber • Mark a fired bullet on its base for identification • Do not mark empty cartridges on base for identification • Make accurate measurements of exact locations where cartridges were found

  27. Tool Marks • Photograph • Do not attempt to make test marks with weapon at crime scene • Make casts of tool mark impressions • Package all items individually

  28. Shoe Prints • Class Characteristics • Brand • Color • Size • Individual Characteristics • Wear pattern • Flaws or other marks

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